'promise' return empty array

Question

I am trying to return Array of tokens stored in Firebase, and I am using 'promise'.

function getUsersTokens() {
 let dbRef = db.ref('/system/users');
 let result = new Promise((resolve, reject) => {
    dbRef.once('value', (snap) => {
        let tokens = [];
        snap.forEach(child => {
            if(child.Status != "occupied"){
                helper.getToken(child.key,db).then(function(token){
                    tokens.push(token);
                });
            }       
        }); 
        resolve(tokens);
    }, (err) => {
        reject(err);
    });
 });
 return result;
  }

and this is the 'getToken' method from the "helper" module.

exports.getToken=function(uid,db){
   return db.ref(`/Tokens/${uid}`).once('value').then(function(result){
     return result.val();
   });
};

The problem is that every time I push token into the array it all works fine, but when exit getUsersTokens() the array gets empty.

thanks for the help.

Christian Santos · Accepted Answer · 2017-12-01 15:21:21Z

The issue is that your result promise is resolving too early because the helper.getToken() is non-blocking, so your forEach will finish running before all of the getToken() calls have finished pushing their token into tokens.

To make things a little easier, you can split your result promise into two promises. The first promise will be in charge of getting snap. The second promise will be in charge of iterating through snap to produce an array of tokens:

function getUsersTokens() {

    let dbRef = db.ref('/system/users');

    let result = new Promise((resolve, reject) => {
        dbRef.once('value', (snap) => {
            resolve(snap);
        }, (err) => {
            reject(err);
        });
    });

    return result.then(snap => {

        let prommiseArr = [];

        snap.forEach(child => {
            if(child.Status != "occupied"){
                let p = helper.getToken(child.key,db);
                promiseArr.push(p);
            }  
        });

        return Promise.all(promiseArr); // resolves to array of tokens

    });
}

Promise.all takes in an array of promises, and resolves when all of those promises have also resolved. the promise returned by getUsersToken will ultimately contain an array of tokens, because each promise of promiseArr resolves to a token.

André Werlang · Accepted Answer · 2017-12-01 15:21:29Z

1

It happens because the promise is resolved with the token array before getToken() resolves itself. You see an empty array because your handler runs before the tokens arrive.

You need to wait on that before resolving. Like this:

function getUsersTokens() {
  let dbRef = db.ref('/system/users');
  return new Promise((resolve, reject) => {
    dbRef.once('value', (snap) => {
      const tokensPromise = snap
        .filter(child => child.Status !== "occupied")
        .map(child => helper.getToken(child.key, db));
      resolve(Promise.all(tokensPromise));
    });
  });
}

answered Dec 1, 2017 at 15:21

André Werlang

5,9841 gold badge37 silver badges51 bronze badges

Comments

Stamos · Accepted Answer · 2017-12-01 15:31:24Z

1

Promise.all as pointed out by @André Werlang and @Christian Santos it perfect here is an example using reduce way

function getUsersTokens() {
 let dbRef = db.ref('/system/users');
 let result = new Promise((resolve, reject) => {
    dbRef.once('value', (snap) => {
       snap.reduce((chain, child) => {
          return chain.then(array => {
             return helper.getToken(child.key,db).then(function(token){
                return array.push(token);
              });
          });
       }, Promise.resolve([])).then(tokens=>{
                    resolve(tokens);
                });
    }, (err) => {
        reject(err);
    });
 });
 return result;
  }

answered Dec 1, 2017 at 15:31

Stamos

4,0181 gold badge25 silver badges48 bronze badges

1 Comment

André Werlang Over a year ago

it wouldn't work because tokens are pushed onto the array at a later time. also Promise.resolve([]) is tricky and relies on the fact that resolving a promise isn't immediate.

Collectives™ on Stack Overflow

'promise' return empty array

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related